Sky and Telescope - November 2015 - 19

VARIEGATED PALETTE Left: Mission scientists combined four images from New Horizons' Long Range Reconnaissance Imager (LORRI) with color data from the Ralph
instrument to create this enhanced-color global view of
Pluto. It reveals details as small as 2.2 km. The overall reddish hues are likely due to veneers of organic compounds.

NASA / JHU-APL / SWRI

stunning - both in their geologic diversity and in their
disparate appearance. Dynamicists strongly suspect that
these two worlds have a shared history: Charon likely
assembled from debris thrown out when Pluto suﬀered a
titanic impact eons ago. Yet, despite this linked lineage,
the two worlds couldn't be more diﬀerent.
Let's start with Pluto. A patchwork of dark features
girds much of its equator. Sandwiched between them is
a bright, heart-shaped region that became evident many
months before the spacecraft's arrival. In fact, planetary
scientists had already mapped (thanks to decades of
ground-based observations and Hubble Space Telescope
images) the major bright and dark regions around its
midsection. The terrain surrounding Pluto's north
pole, tipped 51°° toward the Sun right now, appears to be
frosted with frozen molecular nitrogen (N2) and methane (CH4). This strong axial tilt also means that most of
the southern hemisphere remained hidden in shadow
throughout the encounter.
The heart-shaped expanse, provisionally named
Tombaugh Regio to honor Pluto's discoverer, is about
1,600 km across. "We could see the heart from far away,

NASA / BILL INGALLS

MOTTLED MOON Above: Although it and Pluto orbit just
19,596 km (12,176 miles) apart, center to center, the sizable
moon Charon has a very dissimilar surface from its dwarf
planet. Shown here as seen by New Horizons on July 13th,
the moon hosts an array of landforms - including a dark,
red-tinged polar cap - that has stunned mission scientists.

70 million miles, shining like a beacon as the brightest
feature on the planet," explains principal investigator
Alan Stern (Southwest Research Institute). "So that's
why we want to call it Tombaugh Regio." (As of late
August, the International Astronomical Union hadn't
oﬃcially approved the provisional names used here.)
As the spacecraft closed in, its images showed that the
heart's western half is very smooth and ﬂat - hardly the
ancient, cratered landscape that most everyone expected.
The best views of this Texas-size region, dubbed Sputnik
Planum, reveal meandering, interconnected depressions
that subdivide the terrain into crude polygons roughly 30
to 60 km across. But what the team found most astonishing is the utter lack of impact craters there. According to
Jeﬀ Moore (Southwest Research Institute), this means the
surface of Sputnik Planum can't be more than 100 million years old - and it's likely much younger. "It could be
a week old, for all we know," he says.
Maybe there aren't any craters because the surface
keeps moving around. One close-up shows that icy ﬂows
have migrated northward into a much older, eroded, heavily cratered, and generally beat-up-looking terrain. These
ﬂows mimic the motions of glaciers on Earth, but they
can't be slabs of water ice. At Pluto's cryogenic surface
temperature, roughly 38 kelvin (-390°F), water ice is about
as mobile as the Rock of Gibraltar. Instead, the ﬂows
must consist of frozen nitrogen, methane, and carbon
monoxide (CO), all of which remain pliable even in Pluto's
extreme cold. In fact, one of New Horizons' spectrometers
found that the CO ice is concentrated only within Sputnik
Planum - curiously, it's nowhere else on Pluto.
Two clusters of rugged mountains, Hillary Montes
and Norgay Montes (named for the ﬁrst climbers to
reach the summit of Mount Everest), frame the southern
rim of Sputnik Planum and rise up to 3½ km (11,000
feet) in spots. That's far too tall, notes John Spencer
(Southwest Research Institute), to be big piles of frozen
nitrogen and methane. Instead, the underlying "rock"
must be stiﬀer, sturdier water ice - something long

TIME TO CELEBRATE Guests and mission personnel count down to
the moment when New Horizons passed closest to Pluto, at the Johns
Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Sk yandTelescope.com November 2015

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Sky and Telescope - November 2015

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